Prior application Ser. No. 246,930, filed Apr. 24, 1972, now abandoned, by William Oroshnik, assigned to assignee of the present application, describes a new process for making Vitamin A, Vitamin byproducts of Vitamine A, and isomers thereof. One of the intermediates employed in the method of the application is isoprene chloroacetate (1-chloro-4-acetoxy-2-methyl-2-butene).
In addition to being useful in making Vitamin A and related products, the isoprene chloroacetate may also be a valuable intermediate in the synthesis of Vitamin E, in the synthesis of carotenoids and in the synthesis of many other terpenic and non-terpenic materials in which isoprene is a basic unit in the molecule. (See copending application Ser. No. 560,550, filed Mar. 20, 1975, by Ralph E. Close and William Oroshnik, on "Synthesis of Dehydrophytol and Vitamin E".)
The synthesis of isoprene-1,4-chloroacetate is known and is reported in the Journal of the American Chemical Society, Vol. 72, pages 4608-13, (1950) in an article by W. Oroshnik and R. A. Mallory. The synthesis involves the treatment of isoprene with tert-butyl hypochlorite in the presence of a large excess of acetic acid, and gives the desired 1,4-chloroacetate; namely the cis and trans forms of 1-chloro-4-acetoxy-2-methyl-2-butene (formula I above). Although not reported in the above article, this synthesis also gives a small amount of 4-chloro-1-acetoxy-2-methyl-2-butene (formula II) as follows: ##STR2## Unfortunately, the process also yields the 1,2 chloroacetate isomer, 1-chloro-2-acetoxy-2-methyl-3-butene (formula III), and a mixture of chloroethers, 1-chloro-4-tert-butoxy-2-methyl-2-butene (formula IV), and 4-chloro-1-tert-butoxy-2-methyl-2-butene (formula V). ##STR3## It was reported in the aforementioned article that the ratios of 1,4-chloroacetate (I) to 1,2-chloroacetate (III) to ethers (IV and V) was 32 : 20 : 5, with the ether (V) predominating over the ether (IV). The article further reported that the ethers could be converted using acetic anhydride and anhydrous FeCl.sub.3, to the corresponding chloroacetates (I and II), respectively; and the 1,2 chloroacetate (III) could be converted to the 1,4-chloroacetate by treatment with glacial acetic acid containing sulfuric acid and copper sulfate (CuSO.sub.4 . H.sub.2 O). If the catalyst was omitted, no reaction was observed.
A primary disadvantage with the Oroshnik and Mallory process resides in the additional process steps required to obtain a good yield of the desired 1,4-compounds; namely, at the very least, distillation of the crude product of the hypochlorite/isoprene reaction and work-up of 1,4-product, followed by sulfuric acid treatment of remaining 1,2-product and the chloroethers as described, redistillation, and a second work-up. Work-up itself is, of course, a process involving a series of process steps. Accordingly, the steps are numerous and the process is costly.